Quantum computing has been a rapidly developing research field in the past two decades. At the heart of quantum computing is the quantum bit, or qubit. The qubit is a unit of quantum information, the quantum analogue of the classical bit in our current computer systems. Qubits in a quantum computer must be able to retain the quantum information they are given long enough to perform quantum logic operations with them.
In principle, any two-level quantum system can be used as a qubit. A wide range of candidate quantum systems have been studied for their possible implementation in practical quantum computer as qubits. They can be photons, electrons or nuclear spins, trapped atoms or ions, defect quantum states in solids, superconducting circuits (Josephson junctions), etc.
Superconducting circuits with Josephson junction are solid state devices fabricated by modern integrated circuits techniques, and are manipulated and measured by well-developed low frequency electronics and microwave techniques. A Josephson junction is formed by connecting two superconducting electrodes separated by a dielectric insulating layer. A group of qubits that are based on superconducting circuits involves nanofabricated superconducting electrodes coupled through Josephson junctions. Such systems are one of the most promising systems for being fully electronic and easily scalable for large arrays of qubits. See M. Steffan, “Superconducting Qubits Are Getting Serious,” Physics 4, 103 (2011).
There are many different ways to build a qubit, each having its own pros and cons.
Superconducting circuits with Josephson junctions have emerged as a promising technology for quantum information processing with solid-state devices for its scalability, in which superconductor is assembled macroscopically to form qubits. Since such qubits involve the collective motion of a large number (˜1010) of Cooper-pair electrons, the coherence time is typically very short. Progress has been made to increase the coherence time from 1 ns in 1999 to 60 μs in 2011. See Steffan, supra. It is now understood that dielectric loss from two-level states in the dielectric insulating layer is the dominant decoherence source in superconducting qubits. See J. M. Martinis, K. B. Cooper, R. McDermott, Matthias Steffen, M. Ansmann, K. D. Osborn, K. Cicak, S. Oh, D. P. Pappas, R. W. Simmonds, and C. C. Yu, “Decoherence in Josephson Qubits from Dielectric Loss,” Phys. Rev. Lett., 95, 210503 (2005). The solution will be either to reduce TLS in dielectric layer or to minimize their impact by other means.
Currently, the main issue that limits the performance of superconducting qubits is the decoherence caused by spurious coupling of qubits to microscopic defect states in the materials used to implement the circuits. Dielectric loss from the two-level tunneling systems (TLS) in the amorphous dielectric thin films used as insulating layers is the dominant source of decoherence. See Martinis, supra. TLS universally exist in almost all kinds of amorphous solids and a large number of disordered crystalline solids. R. O. Pohl, X. Liu, and E. J. Thompson, “Low temperature thermal conductivity and acoustic attenuation in amorphous solids,” Rev. of Mod. Phys. 74, 991 (2002). A special type of hydrogenated amorphous silicon prepared by hot-wire chemical vapor deposition was found to contain almost no TLS. See X. Liu, B. E. White, Jr., R. O. Pohl, E. Iwanizcko, K. M. Jones, A. H. Mahan, B. N. Nelson, R. S. Crandall, and S. Veprek, “Amorphous solid without low energy excitations,” Phys. Rev. Lett. 78, 4418 (1997) (“Liu 1997”). However, this material is difficult to prepare and the TLS content is hard to control in a reproducible way. X. Liu and R. O. Pohl, “Low-energy excitations in amorphous films of silicon and germanium”, Phys. Rev. B 58, 9067 (1998) (“Liu 1998”).
Efforts have been made to reduce the density of TLS. Hydrogenated silicon nitride has been used to replace silicon dioxide as dielectric layer and dielectric loss is reduced by a factor of 50. See H. Paik and K. D. Osborn, “Reducing quantum-regime dielectric loss of silicon nitride for superconducting quantum circuits,” Appl. Phys. Lett., 96, 072505 (2010). Efforts have also been made to make overall device size larger while keeping the dielectric layer thickness as small as possible to reduce the relative impact. See H. Paik, D. I. Schuster, L. S. Bishop, G. Kirchmair, G. Catelani, A. P. Sears, B. R. Johnson, M. J. Reagor, L. Frunzio, L. I. Glazman, S. M. Girvin, M. H. Devoret, and R. J. Schoelkop, “Observation of High Coherence in Josephson Junction Qubits Measured in a Three-Dimensional Circuit QED Architecture,” Phys. Rev. Lett. 107, 240501 (2011). This has achieved the record long coherence time of 60 μs. Of course, using completely crystalline silicon as dielectric layer has also being pursued with limited success as surface defect states become the main source of dielectric loss. See S. J. Weber, K. W. Murch, D. H. Slichter, R. Vijay, and I. Siddiqi, “Single crystal silicon capacitors with low microwave loss in the single photon regime,” Appl. Phys. Lett. 98, 172510 (2011).